Databases and Codes

Introduction

Safety analysis for a geological repository for radioactive waste, underground disposal of chemotoxic waste and the remediation of abandoned uranium mines share a common aspect: It is essential that any potential migration of toxic constituents into the biosphere has to be predictable reliably. Important input for the various computational codes are generic, i.e. non-site-specific, thermodynamic data concerning speciation and solubility. Once such data are available – and provided an understanding of the related physico-chemical processes – a coupling of restrictive models to long-term safety assessment codes comes into reach. There, a balance between model details and computational performance is still challenging.

Thermodynamic Reference Database THEREDA

The main objective of joint project THEREDA (Thermodynamic Reference Database, click partners logos on right) is the establishment of a comprehensive and internally consistent thermodynamic reference database for the geochemical modeling of near-field and far-field processes occurring in the different rock formations currently under discussion in Germany to host a repository for radioactive waste. [1] THEREDA offers evaluated thermodynamic data for all compounds of elements, which according to the present state of research are dose relevant.

Actinides, Fission and Activation Products:

Pa, Th, U, Np, Pu, Am, Cm

Rb, Sr, Tc, Cs, Sm, Ra

Matrix:

The website www.thereda.de shall serve users as a portal to the database and as an information and discussion platform on issues concerning the database. The quality management is not restricted to providing a quality assured database, the user ought to be enabled to understand correctly and to apply the quality criteria of the thermodynamic reference database by accompanying information.

Sorption Database RES³T

RES³T (Rossendorf Expert System for Surface and Sorption Thermodynamics) [2] is a digitized version of a thermodynamic sorption database as required for the parametrization of Surface Complexation Models (SCM). It is mineral-specific and can therefore also be used for additive models of more complex solid phases such as rocks or soils. A user interface (www.hzdr.de/res3t) helps to access selected mineral and sorption data, to convert parameter units, to extract internally consistent data sets for sorption modeling. Data records comprise of mineral properties, specific surface area values, characteristics of surface binding sites and their protolysis, sorption ligand information, and surface complexation reactions.

Implementation of Thermodynamic Data into Codes

One important natural retardation process for contaminants (e.g. heavy metals, radionuclides) during the transport through surface sediments or rock fissures is sorption onto mineral surfaces. Describing the sorption as realistically as possible is therefore essential in geochemical modeling.

Within the two BMWi founded joint projects with GRS Braunschweig ESTRAL ("Realistic Integration of Sorption Processes in Transport Programs for Long-Term Safety Assessments"; Nos. 02 E 11072A) and WEIMAR ("Further Development of the Smart Kd-Concept for Long-Term Safety Assessment" (Nos. 02 E 11072B) a new modeling approach is developed [4], where more realistic distribution coefficients are calculated as a function of important environmental parameters such as pH value, ionic strength, competing cations and complex forming ligands using PHREEQC [5] and UCODE [6]. Such a mechanistic approach, namely “Smart Kd concept” based on surface complexation models (SCM) and allows to predict variation in sorption as a consequence of changing physicochemical conditions (e.g. due to anthropogenic causes or climatic changes). Required thermodynamic sorption data are taken from RES³T.

The philosophy behind this approach is to compute a-priori multidimensional smart Kd-matrices for relevant contaminants, which are accessible during (reactive) transport simulations with existing transport codes (e.g. the 3D reactive transport code r³t, radionuclides, reaction, retardation, and transport [7] as well as its update d³f++). It is worth mentioning that this basic methodology can be transferred to any other transport code relying on conventional distribution coefficients as well as to any geological formation.